A lithographic printing plate typically comprises an oleophilic image area that is ink receptive during the printing process and a hydrophilic nonimage area that is fountain solution receptive during the printing process. Lithographic printing is a method that utilizes the fact that water and oleophilic ink repel each other: differences in the ink attachment behavior are produced on the surface of the lithographic printing plate by using the oleophilic image areas on the lithographic printing plate as ink receptive areas and using the hydrophilic nonimage areas on the lithographic printing plate as fountain solution receptive areas (areas not receptive to ink). After ink uptake has been brought about only in the image areas, the ink is transferred to the receiving medium, e.g., paper.
A lithographic printing plate precursor (PS plate) comprising an oleophilic photosensitive resin layer (image recording layer) disposed on a hydrophilic support has heretofore been widely used to produce the aforementioned lithographic printing plate. The lithographic printing plate is typically obtained by carrying out platemaking by a method in which the lithographic printing plate precursor is exposed to light through an original image, for example, a lith film, after which the image recording layer corresponding to the image areas remains while the unwanted image recording layer corresponding to the nonimage areas is dissolved and removed by an alkaline developing solution or an organic solvent-containing developing solution to form the nonimage areas by exposing the surface of the hydrophilic support.
The conventional platemaking process for lithographic printing plate precursors has required a step in which, after photoexposure, the unwanted image recording layer corresponding to a nonimage area is dissolved and removed by, for example, a developing solution, and a concern with these separately conducted wet processes has been to render them unnecessary or to simplify them. In particular, attention to the environment has in recent years caused the disposal of the waste solutions discharged in association with wet processes to become a major issue for the industrial sector as a whole, and there has been an even stronger desire to address the aforementioned concern.
In this context, the method known as on-press development has been introduced as a simple and convenient platemaking method. In on-press development, an image recording layer is used that enables the removal of unwanted areas of the image recording layer to be carried out during the normal printing process: after photoexposure, the lithographic printing plate is obtained by removal of the unwanted areas of the image recording layer on the press.
The following are examples of specific methods of on-press development: use of a lithographic printing plate precursor that has an image recording layer that can be dissolved or dispersed in the fountain solution, in the ink solvent, or in an emulsion of the fountain solution and ink; mechanical removal of the image recording layer by contact with the blanket or rollers on the press; and mechanical removal of the image recording layer by carrying out contact with rollers or the blanket after the cohesive strength within the image recording layer or the adhesive force between the image recording layer and support has been weakened by penetration by, for example, the fountain solution or the ink solvent.
A method known as gum development has also been introduced as another convenient development method. In gum development, the unwanted portions of the image recording layer are removed without using a conventional highly alkaline developing solution, but rather using the gum solution known as the finisher that has been used after a conventional alkaline development.
Unless stated otherwise, in the present invention, the “development processing step” refers to a step in which the hydrophilic surface of the support is exposed by the removal of those areas of the image recording layer on the lithographic printing plate precursor that have not undergone photoexposure, wherein this removal is effected by contact with a fluid (typically an alkaline developing solution) using an apparatus (typically an automatic developing apparatus) outside of the press, while “on-press development” denotes a step and a method in which the hydrophilic surface of the support is exposed by the removal of those areas of the image recording layer on the lithographic printing plate precursor that have not undergone photoexposure, wherein this removal is effected by contact with a fluid (typically the printing ink and/or fountain solution) using the press.
The “development processing step” described above includes development, denoted in particular as “gum development”, that uses a gum solution as the developing solution.
On the other hand, digitization technology, in which the image data is electronically processed, stored, and output by a computer, has become widespread in recent years, and a variety of new image output methods made possible by this digitization technology have entered into use. Accompanying this, computer-to-plate technology has been receiving attention; in computer-to-plate technology, the digitized image data is carried by highly convergent radiation, such as laser light, and the lithographic printing plate is directly produced by scanning photoexposure of the lithographic printing plate precursor with this radiation without going through a lith film. As a consequence, the acquisition of lithographic printing plate precursors adapted to this technology has become an important technical issue.
To simplify the previously described platemaking operation, a system that uses a light source and an image recording layer that support handling in a bright room or under yellow illumination is preferred from the standpoint of ease of operability.
Solid-state lasers that emit infrared radiation at wavelengths from 760 to 1200 nm, e.g., semiconductor lasers and YAG lasers, are such a laser light source and are very useful because small, high output solid-state lasers can be acquired inexpensively. UV lasers can also be used.
Within the realm of the lithographic printing plate precursors under consideration, for example, Patent Document 1 describes a lithographic printing plate precursor that has a hydrophilic support and disposed thereon an image-forming layer in which particles of a hydrophobic thermoplastic polymer are dispersed in a hydrophilic binder. This Patent Document 1 essentially states that on-press development by the fountain solution and/or ink is possible after this lithographic printing plate precursor has been photoexposed using an infrared laser to effect image formation by thermally coalescing the hydrophobic thermoplastic polymer particles and has thereafter been mounted on a cylinder in a press.
This image formation method based on coalescence by the simple hot-melt bonding of finely divided particles does provide an excellent on-press developability, but it has had the drawback of an unsatisfactory printing durability due to its very low image strength.
In addition, Patent Documents 2 and 3 describe lithographic printing plate precursors that have a hydrophilic support and disposed thereon microcapsules that incorporate a polymerizable compound.
Patent Document 4 describes a lithographic printing plate precursor comprising a support having disposed thereon a photosensitive layer that contains an infrared absorber, a radical polymerization initiator, and a polymerizable compound.
Due to the high chemical bond density in the image region, these polymerization reaction-based methods characteristically provide a better image strength than does an image region that has been formed by the hot-melt bonding of finely divided polymer particles, but when viewed from a practical perspective, the on-press developability, printing durability, and polymerization efficiency (sensitivity) are still unsatisfactory.
In order to solve this problem, Patent Document 5 describes a lithographic printing plate precursor that has a sulfonate salt and/or an alkyl sulfate ester salt in a photosensitive layer and Patent Document 6 describes a lithographic printing plate precursor that has an amino acid and/or a betaine in a protective layer; however, the balance here between on-press developability and printing durability is unsatisfactory.
In addition, the development of the nonphotoexposed regions has been completely unsatisfactory when these lithographic printing plate precursors have been subjected to gum development.    Patent Document 1: Japanese Patent No. 2,938,397    Patent Document 2: Japanese Patent Application Publication No. 2001-277740    Patent Document 3: Japanese Patent Application Publication No. 2001-277742    Patent Document 4: Japanese Patent Application Publication No. 2002-287334    Patent Document 5: Japanese Patent Application Publication No. 2007-276454    Patent Document 6: European Patent Application Publication No. 1862301 A